Love a great spa day? You probably have Deborah Szekely to thank. Widely credited with founding the mind/body/fitness movement, Szekely opened the famed Rancho La Puerta in Tecate, Baja California, Mexico, back in 1940, with her husband, the late Edmond Szekely. And in 1958 she founded the renowned Golden Door spa in Escondido, California—a haven for celebrities and spa-goers from around the globe.

On May 3 Szekely will celebrate her 90th birthday, but age has not slowed her down. She continues to direct both spas, travels, lectures and is a noted philanthropist and activist. We asked Szekely about starting her businesses and changes she has seen in the industry, and for advice on staying active. An edited version of the interview follows.

MORE: What inspired you to start Rancho La Puerta back in 1940?
Deborah Szekely: We needed a way to make a living! I was newly married, and my husband’s income came from writing and doing health camps. But his visa to the United States expired as the war in Europe raged on [Edmond was Hungarian], so we opened a summer camp here in Tecate, Baja California—the same property we own today. We charged $17.50 a week and told guests to bring their own tents. Ten years later we were only charging $6 a day. It was a long, hard 10 years. Our gym was nature: We had a mountain for guests to climb, a river to swim in, goats to milk and a vegetable garden to till. We were more like a commune than a resort.

MORE: In 1958 you opened the now famous Golden Door spa near San Diego. Why did you decide to expand?
DS: The idea came from our guests at Rancho La Puerta. Some women—several of them celebrities in the motion picture business—told me, “I wish we had a place just for us where we could let our hair down and chat and just be women together.”

MORE: Those celebrities have included the likes of Gloria Swanson, Barbra Streisand and Burt Lancaster. Did that kind of star power help business?
DS: No doubt star power made us. We were a little place in northern San Diego charging $250 a week, which back in the late ’50s was a large sum . . . At that time people knew nothing about fitness. No one had personal trainers. Thanks to our staff, if you were a 28-year-old movie star who wanted to look 18 by the time your next movie started shooting, a few weeks at the Door could accomplish that. Word spread and soon we had just about every starlet. Some came for a month, their stay paid for by their studio. Some came with their voice coach . . . Star power still helps us and other top spas today, but the stars [now] are more movers and shakers than film stars.

MORE: What are the biggest changes you've seen in the spa industry over the years?
DS: The guests get thinner, and fitter and fitter, so fitness programs have had to become much more intense and varied to match the guests’ condition and sophistication. The fitness classes and hikes we offer now are much tougher than 50 years ago. Take hiking, for example. Back then, most of our guests climbed the mountain just to a lower fence line, not far above the main ranch grounds. Now, our guests hike three to five miles or more on challenging trails, gaining 1,000 feet. Or another example: 50 years ago some guests thought yoga was a religion and didn’t want to even try it. Today the classes are always full. We didn’t change yoga, but the guests’ attitude certainly changed. So the biggest difference in the spa industry is the guests themselves.

MORE: Where do you see the spa industry going in the future?
DS: It depends upon technology—great medical breakthroughs that are happening every day—but most of all, upon the clientele. I believe spa-goers are getting healthier and healthier, especially the people coming to us. In other words, we have to always have greater knowledge than our clients . . . they push us to be better and better.